Without limiting the scope of the invention, its background is described in connection with FLT3 tyrosine kinase.
The FMS-like tyrosine kinase 3 (FLT3) gene encodes a membrane bound receptor tyrosine kinase that affects hematopoiesis leading to hematological disorders and malignancies. See Drexler, H G et al. Expression of FLT3 receptor and response to FLT3 ligand by leukemic cells. Leukemia. 1996; 10:588-599; Gilliland, D G and J D Griffin. The roles of FLT3 in hematopoiesis and leukemia. Blood. 2002; 100:1532-1542; Stirewalt, D L and J P Radich. The role of FLT3 in hematopoietic malignancies. Nat Rev Cancer. 2003; 3:650-665. Activation of FLT3 receptor tyrosine kinases is initiated through the binding of the FLT3 ligand (FLT3-L) to the FLT3 receptor, also known as Stem cell tyrosine kinase-1 (STK-1) and fetal liver kinase-2 (flk-2), which is expressed on hematopoietic progenitor and stem cells.
FLT3 is one of the most frequently mutated genes in hematological malignancies, present in approximately 30% of adult acute myeloid leukemias (AML). See Nakao M, S Yokota and T Iwai. Internal tandem duplication of the FLT3 gene found in acute myeloid leukemia. Leukemia. 1996; 10:1911-1918; H Kiyoi, M Towatari and S Yokota. Internal Tandem duplication of the FLT3 gene is a novel modality of elongation mutation, which causes constitutive activation of the product. Leukemia. 1998; 12:1333-1337; P D Kottaridis, R E Gale, et al. The presence of a FLT3 internal tandem duplication in patients with acute myeloid leukemia (AML) adds important prognostic information to cytogenetic risk group and response to the first cycle of chemotherapy: analysis of 854 patients from the United Kingdom Medical Research Council AML 10 and 12 trials. Blood. 2001; 98:1742-1759; Yamamoto Y, Kiyoi H, Nakano Y. Activating mutation of D835 within the activation loop of FLT3 in human hematologic malignancies. Blood. 2001; 97:2434-2439; Thiede C, C Steudel, Mohr B. Analysis of FLT3-activating mutations in 979 patients with acute myelogenous leukemia: association with FAB subtypes and identification of subgroups with poor prognosis. Blood. 2002; 99:4326-4335.
The most common FLT3 mutations are internal tandem duplications (ITDs) that lead to in-frame insertions within the juxtamembrane domain of the FLT3 receptor. FLT3-ITD mutations have been reported in 15-35% of adult AML patients. See Nakao M, S Yokota and T Iwai. Internal tandem duplication of the FLT3 gene found in acute myeloid leukemia. Leukemia. 1996; 10:1911-1918; H Kiyoi, M Towatari and S Yokota. Internal Tandem duplication of the FLT3 gene is a novel modality of elongation mutation, which causes constitutive activation of the product. Leukemia. 1998; 12:1333-1337; H Kiyoi, T Naoe and S Yokota. Internal tandem duplication of FLT3 associated with leukocytosis in acute promyelocytic leukemia. Leukemia Study Group of the Ministry of Health and Welfare (Kohseisho). Leukemia. 1997; 11:1447-1452; S Schnittger, C Schoch and M Duga. Analysis of FLT3 length mutations in 1003 patients with acute myeloid leukemia: correlation to cytogenetics, FAB subtype, and prognosis in the AMLCG study and usefulness as a marker for the detection of minimal residual disease. Blood. 2002; 100:59-66. A FLT3-ITD mutation is an independent predictor of poor patient prognosis and is associated with increased relapse risk after standard chemotherapy, and decreased disease free and overall survival. See F M Abu-Duhier, Goodeve A C, Wilson G A, et al. FLT3 internal tandem duplication mutations in adult acute myeloid leukemia define a high risk group. British Journal of Haematology. 2000; 111:190-195; H Kiyoi, T Naoe, Y Nakano, et al. Prognostic implication of FLT3 and N-RAS gene mutations in acute myeloid leukemia. Blood. 1999; 93:3074-3080.
Less frequent are FLT3 point mutations that arise in the activation loop of the FLT3 receptor. The most commonly affected codon is aspartate 835 (D835). Nucleotide substitutions of the D835 residue occur in approximately 5-10% of adult AML patients. See D L Stirewalt and J P Radich. The role of FLT3 in haematopoietic malignancies. Nature Reviews Cancer. 2003; 3:650-665; Y Yamamoto, H Kiyoi and Y Nakano, et al. Activating mutation of D835 within the activation loop of FLT3 in human hematologic malignancies. Blood. 2001; 97:2434-2439; C Thiede, Steudal C, Mohr B, et al. Analysis of FLT3-activating mutations in 979 patients with acute myelogenous leukemia: association with FAB subtypes and identification of subgroups with poor prognosis. Blood. 2002; 99:4326-4335; U Bacher, Haferlach C, W Kern, et al. Prognostic relevance of FLT3-TKD mutations in AML: the combination matters-an analysis of 3082 patients. Blood. 2008; 111:2527-2537.
The heightened frequency of constitutively activated mutant FLT3 in adult AML has made the FLT3 gene a highly attractive drug target in this tumor type. Several FLT3 inhibitors with varying degrees of potency and selectivity for the target have been or are currently being investigated and examined in AML patients. See T Kindler, Lipka D B, and Fischer T. FLT3 as a therapeutic target in AML: still challenging after all these years. Blood. 2010; 116:5089-102.
FLT3 inhibitors known in the art include Lestaurtinib (also known as CEP 701, formerly KT-555, Kyowa Hakko, licensed to Cephalon); CHIR-258 (Chiron Corp.); EB10 and IMC-EB10 (ImClone Systems Inc.); Midostaurin (also known as PKC412, Novartis AG); Tandutinib (also known as MLN-518, formerly CT53518, COR Therapeutics Inc., licensed to Millennium Pharmaceuticals Inc.); Sunitinib (also known as SU11248, Pfizer USA); Quizartinib (also known as AC220, Ambit Biosciences); XL 999 (Exelixis USA, licensed to Symphony Evolution, Inc.); GTP 14564 (Merck Biosciences UK); AG1295 and AG1296; CEP-5214 and CEP-7055 (Cephalon). The following PCT International Applications and U.S. patent applications disclose additional kinase modulators, including modulators of FLT3: WO 2002032861, WO 2002092599, WO 2003035009, WO 2003024931, WO 2003037347, WO 2003057690, WO 2003099771, WO 2004005281, WO 2004016597, WO 2004018419, WO 2004039782, WO 2004043389, WO 2004046120, WO 2004058749, WO 2004058749, WO 2003024969 and U.S Patent Application No, 20040049032. See also Levis M, KF Tse, et al. 2001 “A FLT3 tyrosine kinase inhibitor is selectively cytotoxic to acute myeloid leukemia blasts harboring FLT3 internal tandem duplication mutations.” Blood 98(3): 885-887; Tse K F, et al., Inhibition of FLT3-mediated transformation by use of a tyrosine kinase inhibitor. Leukemia. July 2001; 15 (7): 1001-1010; Smith, B. Douglas et al., Single agent CEP-701, a novel FLT3 inhibitor, shows biologic and clinical activity in patients with relapsed or refractory acute myeloid leukemia Blood, May 2004; 103: 3669-3676; Griswold, Ian J. et al., Effects of MLN518, A Dual FLT3 and KIT Inhibitor, on Normal and Malignant Hematopoiesis. Blood, November 2004; 104 (9): 2912-2918 [Epub ahead of print July 8]; Yee, Kevin W. H. et al., SU5416 and SU5614 inhibit kinase activity of wild-type and mutant FLT3 receptor tyrosine kinase. Blood, October 2002; 100(8): 2941-2949. O'Farrell, Anne-Marie et al., SU11248 is a novel FLT3 tyrosine kinase inhibitor with potent activity in vitro and in vivo. Blood, May 2003; 101(9): 3597-3605; Stone, R. M et al., PKC-412 FLT3 inhibitor therapy in AML: results of a phase II trials. Ann. Hematol. 2004; 83 Suppl 1:589-90; and Murata, K. et al., Selective cytotoxic mechanism of GTP-14564, a novel tyrosine kinase inhibitor in leukemia cells expressing a constitutively active Fms-like tyrosine kinase 3 (FLT3). J Biol. Chem. Aug. 29, 2003; 278 (35): 32892-32898 [Epub 2003 Jun. 18]; Levis, Mark et al., Small Molecule FLT3 Tyrosine Kinase Inhibitors. Current Pharmaceutical Design, 2004, 10, 1183-1193.
The aforementioned inhibitors have either been or are currently being investigated in the preclinical setting, or phase I and II trials as monotherapy in relapsed AML, or in phase III combination studies in relapsed AML. Despite reports of successful inhibition of FLT3 with these compounds in preclinical studies, complete remissions have rarely been achieved in FLT3 mutant AML patients in the clinical setting. In the majority of patients, the clinical response is short-lived. Response criteria for AML clinical trials are adapted from the International Working Group for AML. See Cheson et al. Revised Recommendations of the International Working Group for Diagnosis, Standardization of Response Criteria, Treatment Outcomes, and Reporting Standards for Therapeutic Trials in Acute Myeloid Leukemia. J Clin Oncol. 2003; 21: 4642-4649. Responders are patients who obtain a Complete Response (CR), Complete Response with incomplete blood count recovery (CRi), or Partial Remission (PR). Briefly, criteria are as follows:
1. Complete Remission (CR):                a. Peripheral blood counts:                    i. No circulating blasts            ii. Neutrophil count ≧1.0×109/L            iii. Platelet count ≧100×109/L                        b. Bone marrow aspirate and biopsy:                    i. ≦5% blasts            ii. No Auer Rods            iii. No extramedullary leukemia                        
2. Complete remission with incomplete blood count recovery (CRi):                a. Peripheral blood counts:                    i. No circulating blasts            ii. Neutrophil count <1.0×109/L, or            iii. Platelet count <100×109/L                        b. Bone marrow aspirate and biopsy                    i. ≦5% blasts            ii. No Auer Rods            iii. No extramedullary leukemia                        
3. Partial remission:                a. All CR criteria if abnormal before treatment except:        b. ≧50% reduction in bone marrow blast but still >5%        
To date, clinical responses to FLT3 inhibitors have been primarily limited to clearance of peripheral blood (PB) blasts, which frequently return within a matter of weeks, while bone marrow (BM) blasts remain largely unaffected. For example, treatment with sorafenib, the prior mentioned multi-kinase inhibitor with activity against mutant FLT3, while effective in clearing PB blasts, has resulted in only modest BM blast reductions. See G Borthakur et al. Phase I study of sorafenib in patients with refractory or relapsed acute leukemias. Haematologica. January 2011; 96: 62-8. BM blast percentage plays a central role in the diagnosis and classification of AML. The presence of a heightened percentage of blasts in BM is associated with significantly shorter overall survival. See Small D. FLT3 mutations: biology and treatment. Hematology Am Soc Hematol Educ Program. 2006: 178-84; HM Amin et al. Having a higher blast percentage in circulation than bone marrow: clinical implications in myelodysplastic syndrome and acute lymphoid and myeloid leukemias. Leukemia. 2005; 19: 1567-72. To effectively treat FLT3 mutated AML patients and overcome the significant unmet need in this patient population, an inhibitor that significantly depletes both PB and BM blasts, bridge high risk and heavily pretreated patients to stem cell transplant, and can help to decrease relapse rates and increase overall survival in early stage disease patients. The current invention seeks to overcome disadvantages of the prior art.